1. Field of the Invention
The present invention relates to an internal focus imaging lens having an anti-vibration function and suitable for digital cameras, video cameras, etc.
2. Description of the Related Art
Conventionally, numerous internal focus imaging lenses have been proposed that have an anti-vibration function of preventing image blur that occurs consequent to vibration. These imaging lenses further move a relatively light midsection lens group to perform focusing (see, for example, Japanese Patent Nos. 3745104, 3541283, and 3486541; Japanese Patent Application Laid-Open Publication Nos. 2008-145584 and 2005-321574).
Single lens reflex cameras are equipped with a mechanism that uses a mirror disposed just in front of the film, to reflect light that has passed through the imaging lens and thereby guide the light to the optical viewfinder to make the captured image and viewfinder image coincide. However, recently mirror-less single lens cameras that are significantly smaller in size and omit the reflective mirror that guides the image to the optical viewfinder have appeared. Originally, sub-reflective mirrors that guide the images to a distance measuring apparatus during auto-focusing were also disposed with these reflective mirrors. However, since mirror-less single lens cameras omit both the reflective mirror and the sub-reflective mirror, mainstream auto-focusing by phase difference detection cannot be performed in conventional single lens reflex cameras. Thus, in mirror-less single lens cameras, auto-focusing by contrast detection, which is mainstream among compact digital cameras and video cameras, is employed. However, in this case, with mainstream drive apparatuses such as DC motors and ultrasonic motors in conventional interchangeable lenses for single lens reflex cameras, necessary operations against wobble occurring with contrast detection auto-focusing are difficult. Consequently, drive apparatuses such as step motors used in compact digital cameras and video cameras are necessary.
Nonetheless, step motors have a low drive torque and with respect to a focusing lens group that is in a conventional optical system and that can be driven by a conventional DC motor or ultrasonic motor, numerous disadvantages arise related to the focusing lens group being too heavy to drive. With telephoto lenses, this tendency is strong. The same problems occur concerning the anti-vibration lens group. In particular, with a large diameter telephoto lens, since the lenses forming the anti-vibration lens group have a large diameter, anti-vibration lens group tends to be heavy. If the anti-vibration lens group becomes heavy, driving control at the time of vibration correction becomes difficult. In addition, if the lens diameter becomes large, the aberration that occurs also increases and consequently, to control the aberration, the number of lenses configuring the anti-vibration lens group has to be increased, making the anti-vibration lens group even heavier and creating a vicious circle.
The optical systems disclosed in the patent documents above are no exception and the driven focusing lens group and anti-vibration lens group are configured by multiple lenses and are thus, heavy, making driving control by a step motor, which has a low drive torque, difficult.